248 RADIATION BIOLOGY 



where / — luminous intensity of tiie standard lamp, candles, 



E = illuminance, foot-candles (lumens per square foot) or lux 



(lumens per square meter), and 

 d = distance from filament to detector, feet for foot-candle or 

 meters for lux. 

 Since the inverse-square law is employed, the distance d must always be 

 at least five times the maximum dimension of the filament or the photo- 

 cell, depending upon which is the larger, in order to keep within the 

 3 per cent accuracy of most direct-reading photocell-type meters (see 

 Sect. 3 on Inverse-square Law). The general subject of photometric 

 measurements has been treated by Barrows (1951). 



Color Temperature. A lamp of known color temperature is especially 

 useful as a standard of spectral energy distribution for determining the 

 spectral transmittance or efficiency of monochromators. The various 

 methods of measuring color temperature have been briefly reviewed by 

 Harding (1950). The highest-temperature Planckian-radiator source 

 available for direct color matching is that at the temperature of melt- 

 ing iridium at 2716°K. This color temperature is too low for a standard 

 of spectral energy distribution in the near ultraviolet, and therefore, by 

 means of a blue filter, the U.S. National Bureau of Standards (Judd, 

 1950; Teele, 1955) extrapolates to a color temperature of 2854° ± 8°K 

 (C2 = 14,380) for the calibration of standard lamps to meet the specifi- 

 cations of the International Committee on Illumination for Standard 

 Illuminant A. The color-temperature standard as issued by the Bureau 

 is a 500-w tungsten gas-filled projection lamp designed for 120 v and 

 similar to the photometric standard but operated in the vicinity of 90 v 

 and 400 w. Since this source produces a chromaticity match with that 

 of a Planckian radiator at 2854°K, the spectral-energy-distribution curve 

 within the visible spectrum closely approximates that of a complete radi- 

 ator operating at that temperature. The spectral-energy-distribution 

 curve can be extrapolated into the near ultraviolet and visible by obtain- 

 ing the product at each wave length of (1) the relative energy of a com- 

 plete radiator at the true temperature of the filament, and (2) the spectral 

 emissivity of tungsten for the same temperature, as discussed in Sect. 2 

 under Thermal Sources. The true temperature or average filament tem- 

 perature of the 2854°K color-temperature standard lamp is about 2800°K 

 {Tf). This temperature is obtained by empirically trying various values 

 of filament temperature and plotting the product of the relative energies 

 for a complete radiator Jx and the spectral emissivities for tungsten ex. 

 That temperature is selected for the filament temperature which gives 

 the closest fit to a complete radiator at 2854°K within the Hmits of the 

 visible spectrum. Calculations of this type have been made by Stair 

 (1951). Data for the 2854°K color-temperature standard and the spec- 

 tral emissivity of tungsten are given in Table 3-19. 



